In modern passenger vehicles such as passenger cars, traditional keys have been replaced by electronic key systems such as remote keyless systems (RKE) and passive entry systems (PKE). A typical RKE system has a wireless transmitter and a microcontroller (MCU) embedded in the car keys. The MCU mostly stays in sleep mode to save power. When a car key button is pressed, the MCU wakes up and instructs the transmitter to send out a (for example) 64-bit or 128-bit data stream after carrier modulation. An installed radio frequency (RF) receiver receives the data and forwards it to another MCU that verifies the sender's identity and instructs the mechanism to unlock the doors. Key fobs with multiple buttons can perform several other tasks such as opening the trunk, blinking the lights, triggering an alarm, and even starting the engine. The fob transmitter in RKE solutions typically operates at either 315 or 433.92 MHz.
The digital data stream, transmitted typically between 2.4 and 20 kbits/s, usually consists of a data preamble, a command code, some check bits, and a “rolling code” that ensures vehicle security by altering itself with each use. Without this rolling code, the transmitted signal might accidentally unlock another vehicle or be intercepted by a car thief who could use it to gain entry later.
Several major objectives govern the design of these RKE systems. Like all mass-produced automotive components, they must offer high reliability at low cost. They should minimize power drain in both the transmitter and receiver. In addition, the RKE system designer must juggle receiver sensitivity, carrier tolerance, and other technical parameters to achieve maximum transmission range within the constraints imposed by low cost and minimum supply current.
A typical PKE enables users to unlock a vehicle door or trunk without pressing any buttons. They are based on a low-frequency/radio-frequency (LF/RF) communication link between the fob and the vehicle. LF antennas mounted within the outside mirrors or door handles (for example) initiate communication and can sense multiple fobs at a typical range of 1.5 to 2 m. The keys use rolling security codes for authentication to prevent a key from operating a vehicle not associated with the key. PKE solutions can use a 125-kHz RFID transceiver to authenticate the fob and establish communication via the UHF link. When the user pulls a door handle, the passive entry controller sends an LF challenge to authenticate the driver's fob. The fob then sends an RF response to the controller. If fob recognition is successful, the vehicle automatically opens after a few milliseconds. Vehicles with a PKE system disengage the immobilizer and activate the ignition without the key in the ignition, provided the driver has the key inside the car.
Drivers then can start most vehicles with a PKE system by pressing a starter button or twisting an ignition switch. Also, they can lock these vehicles by pressing a button on one of the door handles, by touching a capacitive area on a door handle, or by simply walking away. The locking method varies between models. In any PKE system, a key fob must be able to measure the LF signal strength usually on three orthogonal axes (x, y, and z) and transmit this information via an RF channel. This signal strength information, also known as received signal strength indicator (RSSI), is collected using the antenna coils connected to the LF receiver. Any data such as a wakeup data pattern (preamble, ID) used as a payload in the protocol is received and passed to the key fob MCU for processing. The LF receiver includes dedicated control logic that can check wakeup signals with very low power consumption.
A backup mode enables the use of the PKE system even when the key fob battery is low, with power supplied to the device via the LF signal. The device's response is then transmitted by modulation of the vehicle's own LF signal. When used in this backup mode, the key fob device must be placed close to the door antenna for entry and exit or in a special area on the dashboard to start the vehicle.